Eärendil's setting foot upon the shores of Aman - the first Man to do so - eventually led to the raising of Númenor from the seas and the simultaneous imposition of the Ban of the Valar. After Ar-Pharazôn and his army landed on the shores of the Undying Lands, violating the Ban, the Valar called upon Eru for divine intervention, triggering the events of the Akallabêth.
Why are the Valar so strongly opposed to the entry into Aman by any Man ever again? To those unfamiliar with the LotR universe, there may appear to be two possible reasons:
- Any Man who enters and dwells in Aman - the Undying Lands - will attain immortality and not die, hence the need to restrict entry to ensure all Man will die and thus receive Eru's Gift of Man as intended.
- The arrival of mortal Man will corrupt Aman, causing it to lose its "Undying" status or enchantment.
Anyone familiar with the story will know these reasons are untrue. Exceptions were made for the mortals Bilbo, Frodo, Sam and Gimli to live out the remainder of their lives in the Undying Lands, and they too will die in time. And throughout all of these there is no indication that Aman loses its Undying properties either.
So why this prohibition? Everything that has transpired were to prevent Man from visiting Arda ever again after Eärendil did so, with increasingly stronger measures being taken. It's not like the Valar made exception for the aforementioned on their own initiative: Gandalf had to argue for the Ring-Bearers to be let into the Undying Lands, and Galadriel for Gimli. Their entry did not affect anything as far as we know.